A sealing device for rotation and rolling has screw projections on a seal lip sometimes. This is because the screw pump effect produced by the screw projections can enhance sealing ability.
FIG. 8 shows the construction of a conventional sealing device disclosed in a patent document 1. This sealing device has a seal lip 101 having a wedge-shaped cross section. Screw projections 103 are formed on the atmosphere side slope surface 102 of the seal lip 101. The screw projection 103 is formed in the shape of a combination of a ship bottom screw portion 103a shaped like a ship bottom and a parallel screw portion 103b in which height and width are nearly constant. The parallel screw portion 103b plays the role of securing sealing ability at an early stage and the ship bottom portion 103a plays the role of preventing a decrease in the sealing ability during the progress of wear of the seal lip 101.
FIG. 9 shows the construction of a conventional sealing device disclosed in a patent document 2. The seal lip 201 of this sealing device has a sliding surface 204 extending along the peripheral surface of a shaft 205 between a slope surface 202 on a sealed object side and a slope surface 203 on an atmosphere side. Screw projections 206 are formed from the slope surface 203 on the atmosphere side to the sliding surface 204. The screw projection 206 is basically formed in the shape of a ship bottom and the end portion on the sliding surface 204 side is formed in a shape cut out along the sliding surface 204. Moreover, the sliding surface 204 has parallel screws 207.
In general, a sealing device made of a rubber-like elastic body is manufactured by pouring rubber raw material into a molding die and by vulcanizing the material. When screw projections are formed on the seal lip, depressions corresponding to the shape of screw are formed previously in the molding die by pressing a tool such as a cutter onto the molding die. For example, in the case of the screw projection 103 of the sealing device shown in FIG. 8, as shown in FIG. 10, a depression 301 corresponding to the parallel screw portion 103b is formed by a cutter 300 having a straight cutting edge and then a depression 303 corresponding to the ship bottom screw portion 103a is formed by a cutter 302 having an arc-shaped cutting edge.
[Patent document 1] Japanese Laid-Open Patent Publication No. 9-42463
[Patent document 2] Japanese Laid-Open Patent Publication No. 2003-254439